At present, extensive studies are being carried out for non-aqueous electrolyte secondary batteries (secondary batteries including a non-aqueous electrolyte), in particular, lithium ion secondary batteries having high energy density at high voltages. Commonly, a lithium-containing transition metal oxide (e.g., LiCoO2) is used as the positive electrode active material of a non-aqueous electrolyte secondary battery, and a carbon material is used as the negative electrode active material. As the non-aqueous electrolyte, a non-aqueous solvent in which a solute is dissolved is commonly used. As the non-aqueous solvent, a cyclic carbonic acid ester, a chain carbonic acid ester, a cyclic carboxylic acid ester or the like is used. As the solute, lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4) or the like is used.
From the viewpoint of improving the characteristics of the non-aqueous electrolyte secondary battery, particularly, the storage characteristics and the cycle characteristics, it is very important to control the interface reaction between the non-aqueous electrolyte and the electrode. Side reactions between the non-aqueous electrolyte and the electrode tend to occur especially at high temperatures. When the non-aqueous electrolyte is oxidatively decomposed or reductively decomposed on the surface of the electrode, a large amount of gas is generated to cause buckling (deformation) of the electrode group, so that there is the possibility that a short circuit may occur between the positive electrode and the negative electrode. When a gap is created between the positive electrode and the negative electrode owing to the gas generation, the charge/discharge reaction is impeded. Furthermore, when a side reaction occurs at the interface with the electrode, the impedance is increased, which significantly reduces the storage characteristics.
Attempts have been made to solve such problems by mixing an additive with the non-aqueous electrolyte.
Patent Document 1 suggests including a carboxylic acid vinyl ester such as vinyl acetate in the non-aqueous electrolyte. A carboxylic acid vinyl ester forms a passive film on the surface of a carbon material. The document described that the side reaction between the non-aqueous electrolyte and the negative electrode is therefore suppressed, improving the cycle characteristics and the storage characteristics of the battery.
Patent Document 2 suggests including tert-alkylbenzene such as tert-pentylbenzene in the non-aqueous electrolyte, and describes that this improves the cycle characteristics, the electrical capacity, the storage characteristics and the like of the lithium ion secondary battery.
Patent Document 3 suggests including an anionic-addition polymerizable monomer such as styrene in the non-aqueous electrolyte. An anionic-addition polymerizable monomer forms a film on the surface of a negative electrode comprising a carbon material during charging. The document describes that the side reaction between the non-aqueous electrolyte and the negative electrode is therefore suppressed, improving the cycle characteristics and the storage characteristics of the battery.
Patent Document 1: Laid-Open Patent Publication No. Hei 11-273724
Patent Document 2: Laid-Open Patent Publication No. 2002-298909
Patent Document 3: Laid-Open Patent Publication No. 2000-149989